I would've enjoyed seeing a big-screen Young Indy film, as an interim sort of thing, while waiting for them to get around to Indy IV.
Pick up with Flannery as Indy in his mid-to-latter 20s. Perhaps doing graduate work with Prof Ravenwood, and probably involved with a too-young Marion Ravenwood in a steamy, totally-wrong romance on the side. (Naturally, as how many Indy adventures are there, younger or older, that don't feature a love interest or femme fatale on the side? And This one fills in an untold-tale gap directly related to the setup for Raiders.)
And, if the Young Indy series had continued, I really wanted to see him continue to develop and reach that moment when he donned All the traditional Indy Gear for the First time, embarking on his first mercenary-archeologist-for-hire adventure.
That was my dream last-scene for the Young Indy series. Then you'd leave off, having seen how Young Indy reached that fortune-and-glory stage, and knowing there's several years of adventuring ahead of him where he'll establish his notoriety, until we pick up again in 1935 with Temple Of Doom.
(A Young-Adult Indy film might begin with him about halfway there, with a new-looking leather jacket and satchel. I'd have him lacking only the Whip at first, so we could then find out what life-saving last straw prompts him to add it to his Permament aresenal. And, obviously, learn how to handle it.)
If they'd done a Young Indy film, it was far-enough removed from the era of Harrison's Indy to stand apart. And at the same time, if it were a hit, who knows where they might've gone from there? A couple more films, perhaps. Working around in the Young Indy days, there was a wide-range of options for new stories.
And it's a way of avoiding the issue of trying to Replace Harrison's "Mature" Indy. I'm pretty sure there'll Never be another actor playing Indy in the 30s. But the 20s are still wide open, to my view. You can't replace adult, seasoned Indy. But Young Indy could still be done.
Not sure how old Sean Flannery is now. I haven't seen him in awhile aside from an episode of Dead Zone, where he looked pretty good.
However, considering how much time has passed, it might be best to consider a new actor. Someone who could bridge the gap of Time and Difference between Flannery and Ford's portrayals. (Much the way Flannery bridged the difference between Phoenix and Ford, physically.)
A deeper voice would be good to hear, for example. Don't get me wrong, I think Sean did a great job as Young Indy, I loved the series and enjoyed his performances. He even had the Harrson mannerisms down pat while bringing his own thing to the role as Young Indy. He grew into the hat as time went by.
But sometimes I had a hard time believing his voice would one day turn into that of Harrison Ford's Indy. I used to chalk it up to the assumption that his various failed Romances would accomplish that trick, as he grew more cynical and toughened over the next 15 years, heheh. So, you may wanna look for someone more rugged-sounding if you try to start up a new Indy series set between Young and Raiders.
Anyway, that's an option, the one I'd present.
If you want to do more bigscale Indy films with another actor, I say go the Young-Adult Indy route. The 1920s are still plenty exciting. Supremely so, in some ways. I think Harrison is pretty much irreplaceable in anything set near or beyond 1935.
Because everything beyond Harrison's take on Indy is Based on what Harrison's portrayal Created.
Indiana Jones is inextricable from Harrison Ford...Unless you take the same character significantly Younger and tell the stories that lead Back to that adult version. And then the audience is Still going to be Looking for Traces of Ford in Indy. He's gotta walk that walk.